The simple reason why people don't care as much about the Doctor killing off billions of his own people, while people hate Jack for killing one.

The Doctor's people aren't human, that and it wasn't televised...

But seriously, those people that hate Jack for his actions, what would you of done, all the children were about to be taken to be used in some sick drug trade for the 456, and you have a choice, your grandson, or over a million other children, you could tell he didn't want to do it... But he did what he had to, to quote somebody from a totally different show.

"The needs of the many, outweigh the needs of the few"

In Jack's position, i know i would of done the same thing.

The problem is that you realise it didn't happen? That some guy actually wrote it? That's the biggest problem IMO